In an internal combustion engine, if generating valve overlap between intake valve and exhaust valve when the pressure inside an intake port is higher than the pressure inside an exhaust port, air is blown from the intake passage through the cylinder to the exhaust passage in what is known as “scavenging”. For example, in an internal combustion engine provided with a supercharger such as a turbocharger, scavenging is intentionally caused if the amount of intake air is insufficient for the requested torque. By causing scavenging, the amount of exhaust gas increases and the speed of the turbine of the supercharger is raised. As a result, the pressure of the intake air is raised and the amount of intake air is increased.
Known in the past has been an internal combustion engine configured to provide an air-fuel ratio sensor at the upstream side of an exhaust purification catalyst in an exhaust passage of the internal combustion engine and control the amount of fuel fed to a combustion chamber of the internal combustion engine so that the output of this air-fuel ratio sensor matches a target air-fuel ratio (for example stoichiometric air-fuel ratio (14.6)) (for example, PLT 1).
In such control, during scavenging, the amount of fuel fed to a combustion chamber is controlled so that the average air-fuel ratio of the exhaust gas, including the air expelled from the intake passage through the cylinder to the exhaust passage, becomes a target air-fuel ratio. In this case, during scavenging, the air in the cylinder is decreased by the expulsion of air, so the combustion air-fuel ratio in the cylinder becomes richer than the target air-fuel ratio. Therefore, if valve overlap causes scavenging, in each cylinder, air will be expelled through the exhaust passage in the intake stroke and exhaust gas richer than the target air-fuel ratio will be exhausted into the exhaust passage in the exhaust stroke. In other words, the air-fuel ratio of the exhaust gas exhausted from each cylinder will alternately change between an air-fuel ratio leaner than the target air-fuel ratio and an air-fuel ratio richer than the target air-fuel ratio. In this case, the average air-fuel ratio of the exhaust gas exhausted from all of the cylinders will never greatly deviate from the target air-fuel ratio.